We have conducted a search for “dust chimneys” in a sample of 10 highly-inclined spiral
galaxies () which we had previously observed in the Hα emission line
(Rand 1996). We have procured B-band CCD images for this purpose and employed
unsharp-masking techniques to accentuate the structure of the dust lane. A
scattering+absorption radiation transfer model enabled us to separate 5 galaxies from the
sample which are sufficiently inclined () for us to reliably identify and
quantify dust clouds residing at over 2 scale-heights above the disk. Three of these galaxies
possess numerous curvi-linear chimney structures stretching up to 2 kpc from the midplane and
the fraction of total galactic dust contained in such structures is of order 1%. Optical extinction offers a lower limit to the amount of dust contained in the extraplanar layer but, by examining the transparent submm thermal emission from NGC
891, we fix an upper limit of 5%. Our results are consistent with a similar recent study by Howk & Savage (1999) which indicates that about half of quiescent spiral disks possess detectable dust chimneys. We have compared our optical images with the corresponding Hα emission-line radiation.
We do not find a detailed spatial correspondance between dust chimneys and either sites
of recent star-formation or the extraplanar diffuse ionized gas. This is somewhat surprising given that FIR-bright galaxies, such as
M 82, are known to entrain dust at the working surface of the starburst-driven outflow
(traced in Hα). It is possible a global correlation exists, with disks
experiencing overall higher rates of star-formation also possessing the greatest number of chimneys. This may indicate a timescale difference between the two phenomena with the Hα phase lasting yr but chimneys requiring yr to form. Additionally, we have investigated the edge-on disk NGC 55 which, being ten times closer than
galaxies in our main sample, allows us to examine in greater spatial detail the relationship
between chimneys and recent star-formation. Our discussion touches upon high latitude dust
and supershells observed in the Milky Way. We rule out quiescent disks as prolific
sources of intergalactic grains and metals but note that the rate at which dust is expelled from the main dust layer is comparable to the rate at which it is produced by disk stars (suggesting that it may be an important regulatory process).